1. Field of the Invention
This invention relates to the field of electromagnets. More specifically, the invention comprises a magnet capable of producing an approximately conical field.
2. Description of the Related Art
The present invention proposes to create an electromagnet having a conical bore and, consequently, an approximately conical field. Several approaches may be useful for constructing such a magnet. It is therefore important for the reader to understand some known techniques for electromagnet construction.
A good discussion of prior art magnet construction techniques is found in an article authored by one of the present inventors: Mark D. Bird, “Resistive Magnet Technology for Hybrid Inserts,” Superconductor Science and Technology, vol. 17, 2004, pp. R19-R33. The basic principle of an electromagnet is that a conductor must be wrapped around a central bore for one or more turns. Many turns are typically used. FIG. 3 shows an electromagnet created by wrapping conductor 100 around central bore 104 in a helical path. The two ends of the helical path may be provided with a flat 30 to facilitate mounting the coil. Gaps 28 between adjacent turns on the helical path are typically filled with an insulator of some sort to ensure that the current flows through the helical path. The version shown in FIG. 3 is known as a Florida helix 26. It can be manufactured by cutting the helical gap 28 through a solid cylinder of material, using a wire-EDM process. The resulting conductor is capable of carrying a substantial electric current. This current generates Lorentz forces and considerable heat. Other components are needed to accommodate these factors. Cooling holes or slots traveling parallel to central bore 104 are typically provided. The whole device is placed within a surrounding jacket, so that a pressurized cooling fluid can be pumped through the holes or slots. Mechanical attachment features are generally also provided. For purposes of visual clarity, these features have been omitted.
Bitter-disk type electromagnets are another approach to carrying high currents. While it is true that those skilled in the art are familiar with the design and construction of such magnets, a brief explanation of the prior art will be helpful in understanding the proposed invention.
FIG. 4 shows a prior art Bitter-disk magnet. End plate 40 is the anchoring point for a number of circumferentially-spaced tie rods 44. In practice tie rods 44 have uniform length. Some of these are shown cut away in order to aid visualization of other components. A Bitter-disk magnet is typically constructed by stacking the components. Starting with end plate 40, tie rods 44 are added. A series of conducting disks 36 are then slipped onto tie rods 44. The reader will observe that each conducting disk 36 has a series of holes designed to accommodate tie rods 44. Conducting disks 36 are made of thin conductive material, such as copper or aluminum.
Turning briefly to FIG. 6, the reader may observe conducting disk 36 in more detail. Tie rod holes 46 are uniformly spaced around its perimeter. Cooling holes 54 are also spaced about conducting disk 36.
Cut 52 is a radial cut extending completely through one side of the disk. The reader will observe that the two sides of the disk have been displaced vertically, with the result that conducting disk 36 forms one turn of a helix having a shallow pitch. Upper side 50 of cut 52 is higher than lower side 48. The importance of this fact will become apparent as the construction of the device is explained further.
Prior art Bitter magnets are made in several different ways. The specifics of the prior art construction techniques are not critical to the present invention, since the present invention could be constructed using any of the prior art techniques. However, in order to aid the understanding of those not skilled in the art, one of the prior art construction techniques will be discussed in detail:
Returning now to FIG. 4, the reader will observe that six conducting disks 36 are initially placed over tie rods 44 (the lowest part of the stack in the view). For the specific version shown, as each conductive disk is stacked, it is indexed 1/15 turn in the clockwise direction (corresponding to the fact that there are 15 tie rods 44). Turning to FIG. 7, the effect of the rotational indexing may be more readily observed.
Six conducting disks 36 have been assembled to create one conductor turn 42. Conducting disks 36 have also been “nested” together. The 1/15 turn is a somewhat arbitrary figure. They could be indexed in other increments. Rotational indexing as large as ⅓ turn is in common use, especially for smaller diameter stacks. In fact, it is more customary to divide the 360 degrees found in one complete turn into even increments. If six stacked conductors are used to make one turn, then it would be common to rotationally index each disk ⅙ turn over its predecessor (60 degree index per disk).
The disks are nested in the manner shown, so that upper side 50 of one conductor disk 36 lies over upper side 50 of the conductor disk 36 just below it. The disks in FIG. 4 are shown with a significant gap between them. The Bitter-disk assembly method squeezes the disks tightly together when the device is complete. When squeezed together, conducting disks 36 form one integral conductor having a helical shape—albeit with a very shallow pitch.
Returning now to FIG. 4, the description of the prior art device will be continued. The reader will observe that four conductor turns 42 are shown in the assembly (in the uncompressed state). In reality, many such conductor turns 42 will be stacked onto tie rods 44.
The desired result is to accommodate a large electrical current flowing through a helix having a shallow pitch. The desired path of current flow commences with one end plate 40 (which makes contact with the underside of the lowermost conducting disk 36). A second end plate 40 (not shown) will form the upper boundary of the assembly (“sandwiching” the other components in between). The current will then exit the device through the upper end plate 40 (The tie rods are electrically isolated from the end plates and the disks so that they will carry no current). Those skilled in the art will realize that if one simply stacks a number of conductor turns 42 on the device, the electrical current will not flow in the desired helix. Rather, it will simply flow directly from the lower end plate 40 to the upper end plate 40 in a linear fashion. An additional element is required to prevent this.
Insulating disks 34 are placed within each conductor turn 42 to prevent the aforementioned linear current flow. Each insulating disk 34 is made of a material having a very high electrical resistance. The dimensional features of each insulating disk 34 (tie rod holes, cooling holes, etc.) are similar to the dimensional features of conducting disks 36. Each conductor turn 42 incorporates at least one insulating disk 34 nested into the stack. FIG. 5 shows a detail of this arrangement. The reader will observe the upper portion and lower portion of each insulating disk 34 (both ends of each disk are labeled as “34” in the view so that the reader may easily distinguish them from conducting disks 36). The reader will also observe how each insulating disk 34 nests into the helix formed by the six conducting disks 36.
FIG. 7 also illustrates this arrangement. Insulating disk 34 is placed immediately over the first conducting disk 36. It then follows the same helical pattern as the conducting disk 18. Returning now to FIG. 4, the cumulative effect of this construction will be explained. The four conductor turns 42 shown in FIG. 4 are identical. When they are compressed together, the four insulating disks 34 will form one continuous helix through the stacked conducting disks 36. The insulating disks will then be positioned to form one continuous helical path through the stack. Thus, the construction disclosed forces a helical flow of electrical current through the device. An actual Bitter magnet might include 20 or more such conductor turns.
Those skilled in the art will realize that when a substantial electrical current is passed through Bitter magnet 32, strong mechanical forces are created (Lorentz forces). Significant heat is also introduced through resistive losses. Thus, the device must be able to withstand large internal mechanical forces, and it must also be able to dissipate heat. Once the entire device is assembled with the two end plates 40 in place, the end plates are mechanically forced toward each other. The lower ends of tie rods 44 are attached to the lower end plate 40. The upper ends pass through holes in the upper end plate 40. The exposed upper ends are threaded so that a set of nuts can be threaded onto the exposed ends of tie rods 44 and tightened to draw the entire assembly tightly together. In this fashion, the device is capable of resisting the Lorentz forces, which tend to move the disks and other components relative to each other.
Not all Bitter-type magnets use tie rods. Other mechanical structures can be used to align the components and resist the Lorentz forces. However, since tie rods are the most common approach, they have been illustrated.
Because Bitter magnet 32 generates substantial heat during operation, natural convective cooling is generally inadequate. Forced convective cooling, using deionized water, oil, or liquid nitrogen is therefore employed. A sealed cooling jacket is created by providing an inner cylindrical wall bounded on its lower end by the lower end plate 40, and bounded on its upper end by the upper end plate 42. An outer cylindrical wall is provided outside the outer perimeter of the disks, extending from the lower end plate 42 to the upper end plate 42. All the components illustrated are thereby encased in a sealed chamber. The liquid is then forced into the cooling jacket, where it flows from one end of the device to the other through the aligned cooling holes in the stacked disks (the cooling holes align in the conducting and insulating disks). In FIG. 4, the cooling flow would be linear from top to bottom or bottom to top.
Those skilled in the art will realize that the completed Bitter magnet 32 will generate an intense magnetic field within the cylindrical cavity within the inner cylindrical wall. Those skilled in the art will also realize that it is possible to generate an even greater magnetic field by nesting concentric Bitter-type coils. All these components are well known within the prior art.
The conducting disk shown in FIG. 6 uses round tie rod holes and round cooling holes. Any discontinuity in the cross section of the disk causes structural weakness and imperfections in the magnetic field produced. Viewed only from the standpoint of electromagnetic efficiency, the disk would ideally have no holes at all. Such a design would not work, though, since it could not be effectively cooled. The lack of tie rods would also prevent the disks being effectively aligned and clamped together in order to resist Lorentz forces. Thus, the design of a Bitter-type magnet inherently involves compromises between purity of the magnetic field, conductivity, mechanical strength, cooling, and other factors.
In recent years the traditional Bitter disk design has been improved to remedy some of its shortcomings. FIG. 8 shows a conducting disk developed at the national High Magnetic Field Laboratory in Tallahassee, Fla. This type of disk is now known as a Florida-Bitter disk.
As the tie rods are loaded primarily in tension, a non-round shape can be used. An elongated cross section for the tie rod provides a better compromise between the strength required and the space consumed. Such tie rods are now used. Florida-Bitter disk 56 has elongated tie rod holes 58 to accommodate the modified cross section of the tie rods.
Elongated cooling holes also provide a more advantageous strength versus cooling compromise. Florida-Bitter disk 56 has cooling slots 60 in place of the conventional cooling holes. A series of such cooling slots are placed in rings across the width of the disk.
FIG. 9 shows a detailed view of a portion of Florida-Bitter disk 56, wherein these features can be seen more clearly. The reader will observe that successive circumferential arrays of cooling slots are staggered. If one starts with the innermost array of slots, the next outward array is staggered so that the slots in that array are outboard of the webs in the preceding array. This staggering of the cooling slots substantially enhances the strength of the magnetic field created, and is an important feature of the Florida-Bitter disk.
From these descriptions, the reader will gain some understanding of the construction of high-field resistive magnets. All these techniques can potentially be used in constructing a magnet according to the present invention.